Why Cloud Router Advertises Only Custom Route Ranges

Exhibit

Refer to the exhibit.

```
# gcloud compute routers describe my-router --region us-central1
bgp:
  advertiseMode: CUSTOM
  advertisedGroups:
  - ALL_SUBNETS
  advertisedIpRanges:
  - range: 10.0.1.0/24
  - range: 10.0.2.0/24
bgpPeers:
- interfaceName: if-0
  ipAddress: 169.254.0.1
  peerIpAddress: 169.254.0.2
  peerAsn: 65001
  advertisedRoutePriority: 100
- interfaceName: if-1
  ipAddress: 169.254.1.1
  peerIpAddress: 169.254.1.2
  peerAsn: 65001
  advertisedRoutePriority: 100
```

You run the command shown in the exhibit. Your on-premises network is connected to your VPC via a Cloud Router with two BGP sessions. You notice that your on-premises network receives routes for only the two custom IP ranges (10.0.1.0/24 and 10.0.2.0/24) but not for other subnets in the VPC. What is the most likely cause?

Quick Answer

The answer is that the Cloud Router is configured with custom route advertisement that only includes the two specified ranges. This is because Cloud Router’s BGP advertisement behavior is controlled by its advertisement mode: by default, it advertises all VPC subnets, but when set to custom route advertisement, it advertises only the explicitly listed IP ranges. In this scenario, the on-premises network receives only 10.0.1.0/24 and 10.0.2.0/24 because those are the only custom ranges defined, so other VPC subnets are suppressed from BGP advertisements. On the Google Professional Cloud Network Engineer exam, this tests your understanding of the distinction between the default ALL_SUBNETS mode and custom advertisement, a common trap being that candidates assume all subnets are always advertised. A helpful memory tip is “custom means curated”—if you see only specific ranges in BGP routes, think custom advertisement, not a routing or session failure.

⚠ Common exam trap

A common mix-up: candidates assume BGP session establishment implies all routes are automatically exchanged, but Cloud Router's custom advertisement feature allows granular control over which prefixes are advertised, and the default ALL_SUBNETS behavior is not active when custom ranges are explicitly listed.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The Cloud Router is configured with custom route advertisement that only includes the two specified ranges.

The Cloud Router's custom route advertisement configuration explicitly controls which routes are advertised to on-premises via BGP. If only the two custom IP ranges (10.0.1.0/24 and 10.0.2.0/24) are included in the custom advertisement, other VPC subnets will not be advertised, even if they exist. The exhibit shows that the on-premises network receives only those two ranges, which directly matches a custom advertisement setup rather than the default ALL_SUBNETS behavior.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The BGP interface IP addresses 169.254.x.x are not reachable from the on-premises side.
Why it's wrong here
169.254.x.x are link-local addresses used for BGP peering; they are automatically assigned and should be reachable if the VPN tunnel is up.
✗
The BGP peer ASN 65001 is not recognized by the on-premises router.
Why it's wrong here
ASN 65001 is a valid private ASN; if it were not recognized, BGP session would not establish, but the session is up and routes are received.
✗
The advertisedGroups includes ALL_SUBNETS but the router is ignoring it because of a misconfiguration.
Why it's wrong here
ALL_SUBNETS is present but when mode is CUSTOM, the advertisedGroups is not used; only the custom ranges are used. This is by design, not a misconfiguration.
✓
The Cloud Router is configured with custom route advertisement that only includes the two specified ranges.
Why this is correct
With advertiseMode CUSTOM, the router only advertises the explicitly listed ranges, ignoring ALL_SUBNETS unless it is the only group. The advertisedGroups includes ALL_SUBNETS but since mode is CUSTOM, only custom ranges are advertised.

Visual reference

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Same concept, more angles

1 more way this is tested on PCNE

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A company has a complex on-premises network with multiple BGP AS numbers. They are connecting to GCP using Cloud VPN and wish to advertise specific prefixes. They want to ensure that only selected on-prem prefixes are advertised to GCP and no other prefixes leak. What is the best approach?

hard

✓ A.Use route advertisements from on-prem routers and rely on GCP's route import policy
B.Configure Cloud Router with custom advertised route maps to filter prefixes
C.Use VPC firewall rules to restrict incoming traffic
D.Set up a separate Cloud Router for each prefix

Why A: Cloud Router allows you to configure inbound BGP route filtering using route import policies (BGP prefix filters). By applying these filters, you can specify which on-premises prefixes are accepted, ensuring only selected prefixes are learned by GCP. This prevents unwanted prefix leaks and gives you direct control over what enters your VPC from the on-premises network via Cloud VPN. Custom advertised route maps control outbound advertisements from GCP, not inbound filtering.

Last reviewed: Jun 11, 2026

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